A white light-emitting device is formed by combining, for example, a luminescent material that emits red light by excitation with blue light, a luminescent material that emits green light by excitation with blue light, and a blue LED. For driving of a high-output blue LED, normally a high current of 350 mA or more is introduced, and this driving thus causes the LED to generate heat (for example 135° C. or more). The luminous intensity of a luminescent material generally decreases as the temperature rises (temperature quenching).
By temperature rising, the wavelength of light emitted from a blue LED shifts toward the long wavelength side (for example about 6 nm at 150° C.). If the excitation spectrum of a luminescent material decreases in a wavelength range after the shift, a decrease in luminous intensity becomes noticeable, and it is therefore desired that a decrease in the excitation spectrum of the luminescent material at around a wavelength of light emitted by the LED should be low.
There is a difference in the degree of temperature quenching between a red-emitting luminescent material and a green-emitting luminescent material, and if the temperature of a blue LED rises in a white light-emitting device, the balance between the red and the green is easily broken. As a result, the balance between a red luminescence and a green luminescence from luminescent materials and a blue luminescence from a light source is broken.
A luminescent material having a low decrease in luminous intensity by temperature rising is also desired for avoiding a noticeable “color shift” in a white light-emitting device.